1. Technical Field
The present disclosure relates to a transducer of a MEMS (Micro-Electro-Mechanical System) type, in particular a capacitive microphone, to which the ensuing treatment will make explicit reference, without this implying any loss of generality, and to a corresponding assembly process.
2. Description of the Related Art
As it is usual in this technical field, the term “package” will be used herein to designate, as a whole, the casing, or covering structure, which surrounds, completely or partially, the die or dice of semiconductor material constituting the acoustic transducer, enabling electrical connection thereof to the outside (in particular, connection to a printed circuit of a corresponding electronic device).
As is known, an acoustic transducer, for example a MEMS microphone of a capacitive type, generally comprises a micromechanical sensing structure, designed to transduce acoustic pressure waves into an electrical quantity (in particular a capacitive variation), and reading electronics, designed to carry out appropriate processing operations (amongst which amplification and filtering operations) of this electrical quantity for supplying an electrical output signal (for example, a voltage).
The micromechanical sensing structure in general comprises a mobile electrode, provided as a diaphragm or membrane, set facing a fixed electrode, at a short distance of separation (gap), to provide the plates of a sensing capacitor with a capacitance that varies as a function of the acoustic pressure waves to be detected. The mobile electrode is generally anchored, by means of a perimetral portion thereof, to a fixed structure, whilst a central portion thereof is free to move, or undergo deformation, in response to the pressure exerted by the incident acoustic pressure waves, in this way causing a capacitance variation of the sensing capacitor.
In greater detail, and with reference to FIG. 1, a micromechanical sensing structure of a MEMS acoustic transducer 1, of a known type, comprises a structural layer 2 of semiconductor material, for example silicon, in which a cavity 3 is provided, for example via chemical etching from the back. A membrane, or diaphragm, 4 is coupled to the structural layer 2 and closes the cavity 3 at the top; the membrane 4 is flexible and, in use, undergoes deformation as a function of the pressure of the incident sound waves. A rigid plate 5 (generally referred to as “back-plate”) is set above the membrane 4 and faces it, via the interposition of spacers 6 (for example, made of insulating material, such as silicon oxide). The back plate 5 constitutes the fixed electrode of a sensing capacitor with a variable capacitance, the mobile electrode of which is constituted by the membrane 4, and has a plurality of holes 7, designed to enable the circulation of air towards the membrane 4 (rendering the back plate 5 acoustically transparent). The micromechanical sensing structure further comprises (in a way not illustrated) membrane and back-plate electrical contacts, used for biasing the membrane 4 and the back plate 5 and detecting a signal of capacitive variation resulting from the deformation of the membrane 4 caused by the incident acoustic pressure waves; in general, these electrical contacts are arranged in a surface portion of the die, in which the micromechanical sensing structure is provided.
In a known way, the sensitivity of the MEMS acoustic transducer 1 depends on the mechanical characteristics of the membrane 4 of the micromechanical sensing structure (in particular upon its so-called “mechanical compliance”) and on the type of assembly of the membrane 4 and back plate 5.
In addition, the volume of the front acoustic chamber or simply “front chamber” (i.e., the space traversed in use by acoustic pressure waves coming from the external environment through an appropriate access port), and the volume of the back acoustic chamber, or “back-chamber” (i.e., the space that is located on the opposite side of the front chamber with respect to the membrane 4, set in use at a reference pressure) directly affect the acoustic performance of the transducer.
In particular, the volume of the front chamber behaves as a sort of Helmholtz resonator, on account of the oscillations of the air penetrating through the access port. In fact, the acoustic input signal causes an increase in the pressure of the air inside the front chamber, which consequently acts as a spring pushing out air from the same chamber. As a result of the forces of inertia of the air mass leaving the front chamber, the increase of pressure inside the same chamber is over-compensated, causing a pressure drop, and the negative pressure that is created in the front chamber attracts new air therein. This repeated change of pressure generates the oscillations of air inside the front chamber, at a given resonance frequency. The volume of the front chamber is such as to determine the upper resonance frequency of the acoustic transducer, and hence its performance for high frequencies (in fact, the operative frequency band of the acoustic transducer has to be lower than the resonance frequency of the oscillations of the air): in general, the smaller the volume of the front chamber, the higher the upper cut-off frequency of the transducer in so far as the resonance frequency of the oscillations of air shifts towards higher frequencies.
The back chamber behaves, instead, as a closed volume subject to compression, with the consequence that the smaller the volume of the back chamber, the lower the sensitivity of the acoustic transducer (in fact, it is as if the deformations of the membrane were hindered by the action of a high-stiffness spring). It is hence generally desirable to provide a back chamber of large dimensions so as to improve the sensitivity of the acoustic transducer.
The volume of the front chamber and/or of the back chamber of the MEMS acoustic transducer not only depend upon the configuration of the micromechanical sensing structure, but also upon the conformation of the corresponding package, which has to be configured so as to house not only the same micromechanical sensing structure, but also the reading electronics associated thereto, generally provided as an ASIC in a respective die of semiconductor material.
In the design stage, it has also to be considered that the presence of acoustic access ports, directly communicating with the external environment, designed to enable entry of the acoustic pressure waves towards the membrane 4 of the micromechanical sensing structure, involves the further requirement of pre-arranging appropriate shields for the incident light, which could jeopardize proper operation of the micromechanical sensing structure and of the reading electronics.
Several constraints are thus imposed on the assembly of a MEMS acoustic transducer (and of the corresponding package), which render design thereof particularly problematical, especially where compact dimensions and high electrical and mechanical performance are called for.
In a known assembly arrangement, represented schematically in FIG. 2, a first die 10, integrating the micromechanical sensing structure (here only shown schematically), and a second die 11, integrating the ASIC of the corresponding reading electronics, are coupled side-by-side on a substrate 12. Electrical connections 15 between the first and second dice 10, 11, and between the first die 10 and the substrate 12, are provided with the wire-bonding technique (i.e., with appropriate electrical wires), whilst metallization layers and vias (not shown in detail) are provided through the substrate 12 for routing the electrical signals towards the outside of the package of the MEMS acoustic transducer, which is once again designated as a whole by 1. In a way not illustrated, pads (in the case of an LGA—Land-Grid Array—package), or conductive spherical elements (in the case of a BGA—Ball-Grid Array—package), or similar connection elements, are moreover provided on the underside of the substrate 12 for soldering and electrical connection to an external printed circuit of a corresponding electronic device.
A cap 16 is coupled to the substrate 12, and encloses within it the first and second dice 10, 11. The cap 16 may be made of metal, or of a pre-molded plastic coated within with a metallization layer, in such a way as to prevent disturbance due to external electromagnetic signals (by providing a sort of Faraday cage). The cap 16 is generally attached to the substrate 12 by means of a conductive glue 17 (for example, epoxy resin) so as to obtain also a ground connection towards the substrate 12. The cap 16 further has an opening 18 to enable entry into the package of acoustic pressure waves coming from the external environment.
The above solution is not however free from drawbacks. In particular, the cap 16 is made through molding and hence requires, during production, a set of specific and dedicated molding tools (comprising, for example, dies and punches), for each possible variation of dimensions and shapes that may prove necessary in time, for example following upon the evolution of the dimensions of the silicon structures or upon specific requirements of the end user. In addition, the pitch and layout of the molding and punching tools are not always compatible with the dimensions and configuration of the array of contacts (for example, of an MAP-BGA—Mold-Array Process—Ball-Grid Array—type) each time used for MEMS devices. The production and fixing of the cap 16 to the substrate 12 cannot hence be obtained with technologies and equipment for so-called “mass production”.
The above solution involves large dimensions for accommodating side-by-side the two dice of the MEMS acoustic transducer and for providing the corresponding package, and in general has the disadvantage of not offering to the designer a sufficient freedom (as instead would be desirable) in the sizing of the front and back chambers of the acoustic transducer, for determination of its electrical characteristics. Moreover, in general, providing the electrical connections 15, in particular towards the substrate of the package, can prove problematical in the flow of the assembly process.